Factors chemotactic for neutrophilic leukocytes are being studied. One such factor, derived from culture fluids of Escherichia coli, is being isolated and subjected to chemical studies in an attempt to correlate structure with function. Other chemotactic factors produced when viruses infect cells are also being identified and compared. Finally, chemotactic factors and anaphylatoxins, derived from the complement components C3 and C5, are being compared and contrasted in terms of the characteristics of their generation and their comparative susceptibility to inactivation.